RELATED APPLICATION
The present application is based on Korean Application No. 1758/1997 which is incorporated herein by reference for all purposes.
1. Field of the Invention
The present invention relates to an optical disc and, more specifically, to an optical disc having protective films which enable a high density optical disc for information recording and retrieval.
2. Description of the Related Art
A compact disc (hereinafter, referred to as a CD) and a digital video disc (hereinafter, referred to as a DVD) are representative exemplars of an optical disc, which is a conventional optical recording medium. FIG. 1 depicts the structure of a conventional CD, and FIGS. 2A and 2B depict the structure of a conventional DVD. More specifically, FIG. 2A illustrates the structure of a DVD for a double-sided recording system, and FIG. 2B illustrates the structure of a DVD for a double-layer recording system. The thickness of the CD and DVDs depicted in FIGS. 1, 2A and 2B is approximately 1.2 mm.
The optical disc shown in FIG. 1 comprises a disc substrate 10 carved with pits (made in an uneven form), a reflection film 12 formed in the pits of the disc substrate 10, and a protective film 14 formed on the reflection film 12.
More specifically, FIG. 1 is a sectional diagram of a CD which includes a reflection film 12 of about 0.05 .mu.m in thickness formed in the pits of the disc substrate 10, which is about 1.2 mm thick. A protective film 14, with a thickness of about 10 .mu.m, is deposited on the reflecting film 12.
FIG. 2A is a sectional diagram of a DVD for a double-sided recording system and includes reflection films 24 and 26 which are respectively shaped into disc substrates 20 and 22, each having a thickness of about 0.6 mm. Protective films 28 and 30 are provided on the reflection films 24 and 26 respectively, and an adhesion layer 32 for adhering the above disc substrates 20 and 22 to each other is provided between the protective films 28 and 30. FIG. 2B is a sectional diagram of a double-layer DVD recording system and includes a first reflection film 42 having a thickness of about 0.05 .mu.m shaped in the pit of the disc substrate 40, which has a thickness of about 1.2 mm. A substrate layer 44 having a thickness of approximately 40 .mu.m is formed in the pits of the first reflection film 42. A second reflection film 46, having a thickness of about 0.05 .mu.m, is formed in the pits of the substrate layer 44. A protective film 48, having a thickness of approximately 10 .mu.m, is deposited on the second reflection film 46. In the prior art, the first reflection film 42 is made of a dielectric semitransparent film.
The elements comprising the optical disc illustrated in FIG. 1 and FIGS. 2A and 2B, namely, the disc substrate, the reflection film, and the protective film are made using the following processes and constructed from the following materials. The disc substrate carved with pits is made using processes such as compression, injection, 2 P photo polymerization and so on, and the substrate is made from polycarbonate plastics. The reflection film disposed in the pits of the disc substrate is made through processes such as vacuum evaporation and sputtering and made of aluminum. A protective film is coated on the aluminum reflection film to protect this thin aluminum film from damage which may arise from general physical contact and from oxidation of the reflection film. The spinning process is mainly used to apply this protective film coating, which is generally comprised of a resin material.
An optical disc made using the conventional process above can be contaminated by the adhesion of dust or fingerprints which may be deposited on the optical disc reading surface during use. A minor amount of dust or fingerprints adhering to the reading surface of a conventional optical disc may not produce errors during usage. However, more substantial amounts of dust or fingerprints will produce errors arising from the difficulty of transmitting the energy of a laser beam through the dust and fingerprints to the pits formed in the disc substrate. Moreover, in the case of a high density optical disc, even minor amounts of dust or fingerprints can lead to the same difficulty. The adhesion of fingerprints on the reading surface of an optical disc is an especially well-known problem which can even result in the misreading of information recorded on the disc. Consequently, users are advised to take precautions to avoid permitting dust or fingerprints to adhere to the disc substrate and to remove such substances when they are found on the disc.
Nevertheless, scratches may be generated on the reading surface of an optical disc in the process of removing dust or fingerprints from the reading surface of the optical disc, such that as shown in FIGS. 1, 2A and 2B. In particular, if a fingerprint adheres to the optical disc, the user should certainly clean the surface of the optical disc to remove the fingerprint. Unfortunately, severe scratching may result on the reading surface of the optical disc due to friction caused by the cleansing action. In addition, scratches may easily arise on the reading surface of the optical disc shown in FIGS. 1, 2A and 2B through a user's negligence.
With reference to the optical disc illustrated in FIGS. 1, 2A and 2B, reference numeral 16 (16-1 and 16-2 in FIG. 2A) denotes the reading surface of an optical disc which is made from polycarbonate plastics, as are disc substrates 10, 20, 22 and 40. Polycarbonate plastics offer little protection from scratching.
A cartridge may be used as an external protective device for stably protecting a conventional optical disc, such as that shown in FIGS. 1, 2A, and 2B, against scratching caused during the removal of dust or fingerprints. Unfortunately, the use of a cartridge as an external protective device gives rise to a considerable increase in the manufacturing cost of the optical disc. When the cartridge, serving as the external protection device, is inserted or separated from the disc, the disc can still be seriously exposed to exterior damage. Moreover, there occurs a problem in that scratches may arise on the optical disc due to the external impact caused by simply positioning the optical disc in the cartridge. In summary, the use of a cartridge does not completely protect an optical disc from dust, fingerprints, or scratching.
With optical disc storage capacities rising to higher and higher densities, a solution to the problems above becomes increasingly urgent, especially since high reading data loss per unit area still persists due to dust or fingerprints adhering to the reading surface of the optical disc and scratches made on the reading surface thereof. In other words, while the size of the pits, as a reading unit of the optical disc, steadily decreases to increase the recording capacity of the optical disc, the contamination capable of being generated on the reading surface of the optical disc by various environmental factors, such as dust, fingerprints, and scratches, has not been appreciably reduced. Accordingly, as the recording density of optical disc increases, the negative effects of fingerprints and scratches relatively increase as well.
Error correcting codes can somewhat compensate for the above problems and enable optical disc to be recorded at high density. In fact, the necessity of using error correcting codes to perform error corrections arising from scratches, dust, and fingerprints in high-density pits comparatively increases with higher and higher density. However, in light of other controls, such as the servo, when the width of the recorded track is diminished accordingly, these controls are not increased in proportion to the decrease of the width of the track, which results in deviation from the track.
In other words, if there is an interval where a signal is not detected because of scratches, dust, or fingerprints, which can often arise during ordinary usage, the signal as a controlling object is reduced in proportion to the increase of the recording density in spite of having equal conduction. As a result, it is necessary to heighten the precision of the counterpart servo. In summary, in all applications of optical disc recording and retrieval apparatuses, as the recording density becomes higher and higher, the limitations of error correcting codes to perfectly remove the effects of scratches and fingerprints becomes more pronounced.